Applying a Silane Coupling Agent (17 Kb)

APPLYING A SILANE COUPLING AGENT
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Deposition from aqueous alcohol solutions is the most facile method for preparing silylated
surfaces. A 95% ethanol / 5% water solution is adjusted to pH 4.5–5.5 with acetic acid. Silane is
added with stirring to yield a 2% final concentration. Five minutes should be allowed for
hydrolysis and silanol formation. Large objects, e.g. glass plates, are dipped into the solution,
agitated gently, and removed after 1–2 minutes. They are rinsed free of excess materials by
dipping briefly in ethanol. Particles, e.g. fillers and supports, are silylated by stirring them in
solution for 2–3 minutes and then decanting the solution. The particles are usually rinsed twice
briefly with ethanol. Cure of the silane layer is for 5–10 minutes at 110°C or 24 hours at room
temperature (60% relative humidity).
For aminofunctional silanes such as SIA0610.0 and SIA0589.0 this procedure is modified by
omitting the addition of acetic acid. The procedure is not acceptable for chlorosilanes as bulk
polymerization often occurs. Silane concentration of 2% is a starting point. It usually results in
deposition of trialkoxysilanes as 3–8 molecular layers. Monoalkoxysilanes are always deposited
as monolayers or incomplete monolayers. Caution must be exercised if oven curing. Exhausted,
explosion-proof ovens must always be used.
Deposition from aqueous solution is employed for most commercial fiberglass systems. The
alkoxysilane is dissolved at 0.5–2.0% concentration in water. For less soluble silanes, 0.1% of a
non-ionic surfactant is added prior to the silane and an emulsion rather than a solution is
prepared. If the silane does not contain an amine group the solution is adjusted to pH 5.5 with
acetic acid. The solution is either sprayed onto the substrate or employed as a bath dip. Cure is at
110–120°C for 20–30 minutes.
Stability of aqueous silane solutions varies from hours for the simple alkyl silanes to weeks for
the aminosilanes. Poor solubility parameters limit the use of long chain alkyl and aromatic silanes
by this method. Distilled water is not necessary, but water containing fluoride ions must be
avoided.
Bulk deposition onto powders, e.g. filler treatment, is usually accomplished by a spray-on
method. It assumes that the total amount of silane necessary is known and that sufficient adsorbed
moisture is present on the filler to cause hydrolysis of the silane. The silane is prepared as a 25%
solution in alcohol. The powder is placed in a high intensity solid mixer, e.g. twin cone mixer
with intensifier. The solution is pumped into the agitated powder as a fine spray. In general this
operation is completed within 20 minutes. Dynamic drying methods are most effective. If the
filler is dried in trays, care must be taken to avoid wicking or skinning of the top layer of treated
material by adjusting heat and air flow.
Integral blend methods are used in composite formulations. In this method the silane is used as
a simple additive. Composites can be prepared by the addition of alkoxysilanes or silazanes to
dry-blends of polymer and filler prior to compounding. Generally 0.2 to 1.0 weight percent of
silane (of the total mix) is dispersed by spraying the silane in an alcohol carrier onto a preblend.
The addition of the silane to the non-dispersed filler is not desirable in this technique since it can
lead to agglomeration. The mix is dry-blended briefly and then melt compounded. Vacuum
devolatization of byproducts of silane reaction during melt compounding is necessary to achieve
optimal properties. Properties are sometimes enhanced by adding 0.5–1.0% of tetrabutyl titanate
or benzyldimethylamine to the silane prior to dispersal.

Deposition as a primer is employed where a bulk phase is required as a transition between a
substrate and a final coating. The silane is dissolved at 50% concentration in alcohol. One to three
molar equivalents of water are added. The mixture is allowed to equilibrate for 15-20 mins. then
diluted to 10% concentration with a higher boiling point solvent. Materials to be coated with the
primer are dipped or sprayed and then cured at 110–120°C for 30–45 mins.
Chlorosilanes such as SIV9110.0 may be deposited from alcohol solution. Anhydrous alcohols,
particularly ethanol or isopropanol, are preferred. The chlorosilane is added to the alcohol to yield
a 2–5% solution. The chlorosilane reacts with the alcohol producing an alkoxysilane and HCl.
Progress of the reaction is observed by halt of HCl evolution. Mild warming of the solution (30–
40°C) promotes completion of the reaction. Part of the HCl reacts with the alcohol to produce
small quantities of alkyl halide and water. The water causes formation of silanols from
alkoxysilanes. The silanols condense on the substrate. Treated substrates are cured for 5–10 mins.
at 110°C or allowed to stand 24 hours at room temperature.
Chlorosilanes and silylamines may also be employed to treat substrates under aprotic conditions.
Toluene, tetrahydrofuran, or hydrocarbon solutions are prepared containing 5% silane. The
mixture is refluxed for 12–24 hours with the substrate to be treated. It is washed with the solvent.
The solvent is then removed by air or explosion-proof oven drying. No further cure is necessary.
This reaction involves a direct nucelophilic displacement of the silane chlorines by the surface
silanol. If monolayer deposition is desired, substrates should be predried at 150°C for 4 hours.
Bulk deposition results if adsorbed water is present on the substrate. This method is cumbersome
for large scale preparations and rigorous controls must be established to ensure reproducible
results. More reproducible coverage is obtained with monochlorosilanes.
Silazanes such as SIH6110.0 and SID4612.0 may be used as treatments in concentrated form or
as 10–20% solutions in aprotic solvents. In some applications parts are exposed for 5–10 minutes
by dipping or in microelectronics by spin-on techniques. Optimum reactivity is as 30–50°C. An
alternate method of treatment is to expose parts to 50°C vapor for 2–6 hours. Ammonia is the
byproduct of silazane reaction and areas should be ventilated.
REFERENCES
D. Leyden, W. Collins, “Symposium and Silylated Surfaces” Gordon & Breach 1980.
B. Arkles, “Tailoring Surfaces with Silanes” Chemtech 7, 766 (1977).
E.P. Plueddemann, “Silane Coupling Agents” Plenum N.Y. 1982.
K.L. Mittal “Silanes and Other Coupling Agents” VSP, 1992.
APPENDIX
Calculation of necessary silane to obtain minimum uniform multilayer coverage can be obtained
knowing the values of the specific wetting surface of silane (ws) and the surface area filler
amt. of silane (g) =
Relative surface area of common fillers m 2 /g
amount of filler x surface area of filler
specific wetting surface
E-glass 0.1–0.12 Talc 7
Silica, ground 1–2 Si, diatomaceous 1–3.5
Kaolin 7 Calcium silicate 2.6
Clay 7 Silica, fumed 150–200
Talc 7
Si, diatomaceous 1–3.5

COUPING AGENT RECOMMENDATIONS
THERMOSETS — TABLE 1
Resin Silane Class Gelest Product Code Examples
diallylphthalate amine SIA0589.0 SIA0591.0 SIA0610.0
SIA059.0
styryl
SIS6994.0
epoxy amine SIA0591.0 SIA0610.0 SIT8398.0
epoxy SIG5840.0 SIE4670.0 SIG5832.0
imide amine SIA0610.0 SIA0588.0 SIP6724.0
chloromethylaromatic SIC2295.5 SIC2296.2
melamine amine SIA0589.0 SIA0591.0 SIA0610.0
SIG5840.0 SIE4670.0
paralene chloromethylaromatic SIC2295.5 SIC2296.2
vinyl/allyl SIV9112.0 SIV9220.0 SIA0540.0
phenolic amine SIA0610.0 SIA0591.0 SIA0605.0
epoxy SIG5840.0 SIE4670.0
photoresist, negative silazane SID4612.0 SIH6110.0
aromatic SIP6821.0 SIP6722.6
photoresist, positive silazane SIH6110.1
phosphine
SID4558.0
aromatic SIP6821.0 SIP6722.6
polyester, unsat’d amine SIA0589.0 SIA0591.0 SIA0610.0
methacrylate/vinyl SIM6487.4 SIM6487.5 SIV9112.0
styryl
SIS6994.0
urethane amine SIA0591.0 SIA0610.0 SIT8398.0
epoxy SIG5840.0 SIE4670.0 SIF5832.0
isocyanate
SII6455.0

THERMOPLASTICS — TABLE 2
Resin Silane Class Gelest Product Code Examples
polyacetal quaternary SIS6994.0 SIT8405.0
polyacrylates methacrylate SIM6487.4
ureido / amine SIU9055.0 SIA0610.0
polyamides amine SIA0589.0 SIA0591.0 SIA0610.0
SIA0605.0 SIU9055.0
polyamide-imide amine SIA0610.0 SIA0588.0 SIP6724.0
SIC2295.5 SIC2296.2
polybutylene amine SIA0591.0 SAI0610.0 SIT8398.0
terephthalate isocyanate SII6455.0
polycarbonate amine SIA0591.0 SIA0610.0 SITU9055.0
polyethylene amine SIA0591.0 SIB1833.0 SIT8398.0
vinyl SIV9112.0 SIV9220.0
polyphenylene sulfide mercapto/thio SIM6475.0 SIB1825.0
amine SIA0591.0 SIA0610.0 SIT8398.0
polypropylene styryl SIS6994.0
aromatic SIP6821.0 SIP6722.6 SIP6724.0
polystyrene aromatic SIP6821.0 SIP6722.6 SIP6724.0
polysulfone amine SIA0591.0 SIA0610.0 SITU9055.0
polyvinylbutyral amine SIA0591.0 SIB1833.0 SIT8398.0
polyvinyl chloride amine SIA0591.0 SIB1833.0 SIT8398.0
mercapto/thio SIM6475.0 SIB1825.0
SEALANTS — TABLE 3
acrylics amine SIA0608.0 SIA0591.0 SIA0610.0
methacrylate SIM6487.4 SIM6487.5
polysulfide mercapto/thio SIM6475.0 SIB1825.0 SIT7908.0
amine SIA0591.0 SIB1833.0 SIT8398.0
RUBBERS — TABLE 4
butyl epoxy SIG5832.0 SIE4670.0 SIG5840.0
diene vinyl/cyclic olefin SIB0992.0 SIV9112.0 SIV9220.0
neoprene mercapto/thio SIM6475.0 SIB1825.0 SIT 7908.0
isoprene mercapto/thio SIM6475.0 SIB1825.0 SIT 7908.0
fluorocarbon amine SIA0610.0 SIA0588.0 SIP6724.0
epichlorhydrin amine SIA0591.0 SIB1833.0 SIT8398.0
mercapto/thio SIM6475.0 SIB1825.0 SIT7908.0
silicone allyl/vinyl SIV9112.0 SIV9220.0 SIA0540.0